President Barack Obama's new plan for NASA has been met with
some anger and confusion in the weeks since its announcement, drawing sharp objections
from critics who view it as a radical vision change for the space agency that would
upset the world leadership of the United States in space.

But similar shifts have occurred throughout NASA?s more than
50-year history, and some space experts counter that this new plan  which
would use commercial
spacecraft to fly astronauts in space instead of government spacecraft  is
no more radical than those previous changes.

The new
space plan calls for the cancellation of the existing Constellation
program, which has been overseeing the construction of new Ares I and Ares V
rockets to take humans to low-Earth orbit and back to the moon. Instead, the
Obama administration aims to encourage private industry to develop commercial
spacecraft to ferry humans to and from the International Space Station, while
NASA focuses on research and development to enable future space exploration.

"Of course it's happened in the past," said Roger
Launius, space history curator at the Smithsonian's National Air and Space
Museum. "Of course programs have been canceled  they?ve been canceled in
mid-stream."

For example, he said, some of the hardware to fly the Apollo
18, 19 and 20 missions had already been built when President Nixon cancelled
the Apollo program in the 1970s.

Similarly, Skylab, the United States' first space station,
was meant to be followed up by a sibling station called Skylab B, but that
program was also canceled in the 1970s.

NASA gets radical

In a hearing last week held by members of a House Science
and Technology Committee reviewing President Obama's 2011 NASA budget request,
Senator David Vitter (R-Louisiana) called the proposal a "radical
departure from all previous NASA plans under any administration."

The budget request received vocal opposition from U.S.
senators and House representatives during a series of review hearings last
week. Congressman Bart Gordon (R-Tennessee), chairman of the House Science and
Technology Committee also called the new plan a "radical change" for NASA.

In the $19 billion budget request submitted by President
Obama, NASA would receive a slight boost in funding for 2011. It calls for the
cancellation of NASA?s space shuttle fleet at the end of this year as planned,
but would extend the International Space Station?s lifetime by five years to at
least 2020.

Without the shuttle, and the Constellation program shelved,
NASA will be reliant on Russia?s Soyuz spacecraft to fly Americans to the space
station until commercially built spacecraft are available for ferry flights in
the United States. The gap is expected to take several years.

It is that shift which lawmakers have found hard to stomach.

"It cancels all major existing human spaceflight
programs . . . and replaces all that with little more than a hope and a prayer that
commercial providers will eventually pick up the slack," Vitter said.

NASA administrator Charles
Bolden, who was also at the hearing, countered that the Constellation
program was unsustainable at its current funding level, and that the new
proposal isn't as different as it sounds.

"This is not a radical departure from anything, it's
just a departure from the way we were trying to get there," he said.

In fact, NASA has also faced gaps in access to space for
American astronauts before. The largest spanned six years as NASA shifted from
capsule-based spacecraft to winged and reusable space shuttles.

After the last Apollo spacecraft flew in 1975 to link up
with a Soviet Soyuz in the Apollo-Soyuz Test Project, American astronauts did
not fly in space until April 12, 1981. That was when NASA?s first space shuttle
Columbia blasted off with two astronauts aboard on its first orbital test
flight.

In 1986, the tragic loss of shuttle Challenger and seven
astronauts prompted a two-year hiatus in shuttle flight until safety
improvements could be made. NASA also stood down for two years after the 2003
Columbia accident as well.

Commercial spacecraft details

Fears about the new program also stem from the increased
emphasis on commercially built vehicles. But this concept, too, isn't exactly
new.

"There's been an expectation for many years that the
private sector would become more involved," Launius told SPACE.com. "There's
been efforts across the history of the agency where there's been various pieces
of it they tried to make more private."

Before the Challenger space shuttle accident, there had been
some discussions between NASA and some commercial airline companies about
potentially selling the space shuttle fleet, Launius said. The shuttles would
then have become private vehicles that NASA would have contracted with to provide
rides for astronauts to space.

Those plans were derailed by both the loss of seven
astronauts on Challenger, which exploded and broke apart just after liftoff on
Jan. 28, 1986, and worries by the companies that the deal wouldn't be profitable
enough.

But while there have been intimations of increased
privatization of space, this plan would still represent a new frontier, other
experts say.

"['Radical'] might be a little bit dramatic, but it's
certainly a big shift," said Leroy Chiao, former NASA astronaut and a member
of the blue-ribbon panel President Obama commissioned to review NASA's plans
before designing the new proposal. "I would say it's unprecedented."

He said he thought it made sense to look to commercial
industry to provide transport to low-Earth orbit, but that NASA should also
stay in the business of building spacecraft.

"NASA's job should be focused on exploration, going
beyond low-Earth orbit," he said.

Even though it may be a significant change, Chiao said it
might be for the best.

"Transitions are difficult but sometimes you need some
kind of a dramatic change in order to get that improvement," he said.